1. Field of the Invention
The present invention relates to a heat-developable light-sensitive material, and more particularly, it is concerned with a heat developable light-sensitive material which is practically free from thermal fog (undesirable fog resulting from the heating of an exposed heat-developable light-sensitive material).
2. Description of the Prior Art
Silver halide photography has been much more universally employed in the past, compared with electrophotography, diazo photography and the like, because of the superior photographic characteristics such as sensitivity, gradation, etc., of silver halide photography. However, silver halide photography requires much time and labor, because the silver halide light-sensitive material employed in this method must be subjected to several processings including an image-exposure, a developing process using a developer and processes for preventing the developed image from changing color or deteriorating under normal room-illumination and preventing the non-developed portion (hereinafter background) from blackening, e.g., processing including stop, fixation, washing and rinsing, stabilizing and other similar processes. In addition, the chemical agents which may be used in this method are dangerous to the human body, and the processing room and the workers' hands and clothes are often stained with these agents. Therefore, it has been strongly desired to improve silver halide photography so that the light-sensitive materials can be treated in a dry condition instead of treatment with solutions, and so that the processed images are maintained stable. In order to solve this problem, many efforts have been made.
A first method which has been developed thus far includes the so-called combined developing and fixing bath method wherein two procedures in a conventional silver halide photography, developing and fixing procedures, can be replaced by one procedure, as disclosed in U.S. Pat. No. 2,875,048; British Pat. No. 954,453; and German Patent Application OLS No. 1,163,142. A second method attempts to replace wet procedures in conventional silver halide photography with dry procedures, as disclosed in German Patent Application OLS No. 1,174,159; British Pat. Nos. 943,476 and 951,644; and so on. A third method uses as a main light-sensitive component a silver salt of a long chain aliphatic carboxylic acid such as silver behenate, silver saccharin, silver benzotriazole, etc., and a catalytic amount of a silver halide simultaneously, as disclosed in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,635,719, 3,645,739, and 3,756,829 and Canadian Pat. No. 811,677; and so on.
The present invention belongs to the third method of the above-described three methods.
However, the unexposed parts of the heat-developed light-sensitive materials which have so far been proposed, for example, the unexposed parts of the compositions containing the silver salts of fatty acids such as silver behenate, etc., reducing agents and catalytic amounts of silver halides, become to a considerable extent black when heated (this phenomena is called "thermal fog" hereinafter), which makes the distinction between the images and the background difficult because there is very little contrast between the black images formed on the exposed parts by heating (image density) and the thermally fogged black background. Therefore, a reduction of thermal fog has been an important subject in this art. Moreover, storage of light-sensitive materials for a long time before use under conditions of high temperature (30.degree. C.-50.degree. C.) and high humidity (more than 50% relative humidity) causes fog resulting in the formation of indistinguishable images.
Mercury compounds are known to be effective for improving these disadvantages (as disclosed in U.S. Pat. No. 3,589,903). However, mercury compounds are well known to be toxic and to cause pollution difficulties. For example, heat development of mercury-containing light-sensitive materials results in the diffusion of mercury vapor into the air, and mercury is leached out when such light-sensitive paper are reclaimed as paper.